The present invention relates to displays, and more particularly, to an LED-based backlight for a liquid-crystal display (LCD).
Light-emitting diodes (LEDs) have been proposed for use as a backlighting source for liquid crystal displays (LCDs). Compared to conventional backlights that use fluorescent lamps, LEDs are less expensive, last longer, contribute to a more robust design, and provide a higher brightness-to-size ratio. Specifically, arrays of LEDs emitting white light show promise as LCD backlights.
Some applications require the light emitted from an LCD backlight to possess a specific chromaticity. This is important in fields such as avionics, where the colors displayed on a critical-function aircraft display must be exact. A slight difference in chromaticity of the backlight might adversely affect the color of the display. However, most commercially-available LEDs are produced with a limited number of chromaticity choices.
Furthermore, available LEDs are typically mass-produced and exhibit chromaticities within a certain range. This means that even if the rated chromaticity of an available LED matches the chromaticity requirements for an LCD backlight design, the rated chromaticity may be different from the actual chromaticity. In an array of white LEDs, there may be many individual LEDs that do not match the rated chromaticity, and the total chromaticity of the array may therefore not be predictable.
Another problem is that notwithstanding the long life exhibited by LED""s, there may be some change in LED chromaticity over time. The chromaticity may also be affected by variations in temperature. Additionally, there is the chance that one or more LEDs in an array may be faulty and either fail prematurely or emit light with an unintended chromaticity. In either case the chromaticity of the entire array may be affected. In demanding applications such as avionics it is possible that such a chromaticity change would necessitate untimely repair or removal of the backlight.
It is therefore an object of the invention to extend the range of chromaticities that can be supplied by an LED-based LCD backlight.
It is another object of the invention to compensate for naturally occurring variations in the manufacture of LEDs so that light with a predictable chromaticity is supplied.
It is another object of the invention to extend the useful lifetime of an LED-based LCD backlight.
A feature of the invention is the varying of the brightness of a first set of LEDs relative to the brightness of a second set of LEDs to adjust the chromaticity of the combined light from the first and second sets of LEDs.
An advantage of the invention is that commonly available LEDs may be used in an LCD backlight even if the chromaticity of the commonly available LEDs is different from the desired chromaticity.
The invention provides a backlight for a liquid-crystal display. The backlight includes a first LED array that provides light with a first chromaticity and a second LED array that provides light with a second chromaticity. A combining element combines the light from the first LED array and the second LED array and directs the combined light toward the liquid crystal display. A control system is operationally connected to the second LED array. The controller adjusts the brightness of at least one LED in the second LED array, thereby adjusting the chromaticity of the combined light.
The invention also provides a system for adjusting the chromaticity of an LCD backlight, wherein the backlight provides combined light from a principal LED array and a secondary LED array. The system includes a controller that is operationally connected to the secondary LED array. The controller includes a processor that calculates the chromaticity difference between a target chromaticity of the combined light and an actual chromaticity of the combined light. The controller adjusts the brightness of the secondary LED array to reduce the chromaticity difference.
The invention further provides a method of adjusting the chromaticity of an LCD backlight, wherein the backlight provides light combined from a first LED array and a second LED array. According to the method, a target chromaticity for the LCD backlight is obtained. An actual chromaticity of the LCD backlight is determined. The difference between the target chromaticity and the actual chromaticity is calculated. The brightness of at least one LED in the second LED array is adjusted, thereby changing the chromaticity of the light combined from the first and second LED arrays and decreasing the difference between the target chromaticity and the actual chromaticity.